This invention relates to the burning of waste gas vented away from the site of a petroleum processing operation, including drilling and refining operational sites and, more particularly, to a remote electrical ignition and flame monitoring system and method for maintaining a flare for the burning of waste gas.
The term "waste gases" as used herein is intended to refer to gaseous substances generated during oil drilling and petroleum refining that are of insufficient value to warrant capture or collection. Waste gases include poisonous and explosive products such as hydrogen sulfide and propane, which cannot be permitted to freely escape into the air because of their hazardous and pollutionary nature.
A common method of disposing of waste gases is to burn the gases in the form of a flare as they are generated. The flare can be maintained continuously or produced intermittently depending upon the presence of the waste gases.
Waste gas flares are usually found at chemical plants, refineries, oil and gas well sites, compressor stations, offshore platforms and other locations wherein flammable materials may be discharged into the atmosphere as a by-product of some processing operation.
Due to the hazardous nature of many waste gases, the discharge point of the gases and the location of the flare is preferably at a safe, remote distance from personnel and equipment used in carrying out a drilling and refining process. Thus, waste gases are often discharged and burned at the end of vertical stacks which can rise more than 200 feet above the ground, or from the end of cantilevered venting structures on offshore drilling platforms which can extend approximately 180 feet above water.
Systems for providing waste gas flares at a waste gas disposal point are a vital part of a processing operation, and the failure to provide a flare when needed will usually lead to a total shutdown of a processing or production facility.
It is thus essential that a flare be provided when needed at a waste gas disposal site and that the presence of the flare be monitored when a burning operation is to be carried out. In some processing or refining operations, the discharge of waste gases can occur without warning and it is normal practice under these conditions to have a pilot flame burn continuously, to ignite the gases as they are vented.
A common problem in many known flame monitoring and ignition systems is their occasional failure to clearly indicate whether a pilot flame is present or absent. To offset this difficulty some operators intentionally vent an additional volume of gases through a stack to make a flare more apparent. However such practice can be dangerous to both personnel and equipment because it is difficult to measure the amount of raw gas accumulating in the stack before it reaches the flare point. Explosions can thus result.
One type of flame sensing device in present use includes a heat sensor. However heat sensing devices are subject to slow response, premature burnout due to extreme temperatures, and lightning damage. The reliability of heat sensing devices is thus questionable.
Another known flame sensing device detects the ionization of gas molecules resulting from a burning flame. The sensors used in this device are somewhat fragile and depend upon a separate source of electric current, as well as a separate conductor for operation.
Optical sensors have also been used to monitor the presence of a flare but require sensitive electronic systems that are subject to frequent failure in harsh operating environments.
In some instances, the discharge of waste gases occurs at predictable predetermined times and the ignition of such gases can be arranged to occur at such time as they are discharged.
Thus it is often desirable to have the capability of igniting a flare intermittently at a site remote from operating personnel and equipment.
One known way of igniting a flare on an as-needed basis is to use a flame-front system to ignite a pilot light which, in turn, ignites the waste gas flare. The flame-front system employs a mixture of air and gas which is purged along a pipe from the ground to the flare tip. A spark is then introduced into the pipe at the ground to ignite the gas-air mixture in the pipe. A resulting flame front of burning gases progresses along the pipe to the tip, and exits into the pilot body igniting the main gas supply.
The flame-front system is very sensitive to humidity, rain, piping orientation, and other characteristics of individual situations wherein the pipe is deployed. The flame-front system often requires numerous attempts to ignite a flame successfully and has occasionally been found to be unreliable.
When a flame-front system fails to ignite a pilot flame and the waste-gas flare, a backup procedure, such as the firing of a pyrotechnic flare from a rifle or pistol though the escaping gases may be employed. The use of pyrotechnic flares requires a skillful operator, and is seldom safe in the environment of a refinery or off-shore platform.
Known attempts to resolve the foregoing ignition problems include the use of electronic ignition systems employing a high voltage device such as a transformer to deliver a reliable spark at an igniter head. The term "high voltage" as used herein refers to voltages in excess of approximately several kilovolts. Other circuitry employed in known electronic ignition systems include relaxation oscillators or high voltage generators. Such electrical equipment, while capable of developing high voltage often cannot deliver adequate current for ignition.
Another problem in the use of electronic ignition has been the need for a long ignition cable to bring high voltage to the site of the flare. Such cables have been found to introduce substantial loss to electric signals transmitted along the cables. Consequently, some electronic units are located relatively close to an igniter electrode. Close proximity of an electronic unit to an igniter electrode subjects the electronic circuitry to tremendous amounts of heat which leads to premature failure. Furthermore, since the electronic circuitry is practically inaccessible, it is costly to repair.
A further problem of electronic ignitors is that they operate with high frequency signals, in excess of several kilohertz. Such high frequency signals are attenuated by the capacitance of an ignition cable. Thus, presently available ignition devices deploying ignition cables have a maximum operating distance limit, typically of 25 feet in optimum conditions.
It is thus desirable to provide a flame ignition and monitoring system which reliably monitors the existence of waste gas flare and which can be operated at periodic intervals to provide an ignition spark when needed.